| The samples for this study were taken from reactor pressure vessel (RPV) model steel having higher Cu content. The samples were divided into two sets. The first set of samples were heat treatment of 0.5 h at 880℃and quenched into water, and the second set of samples were tempered at 660℃for 10 h followed by air cooling after an initial heat treatment of 0.5 h at 880℃and quenched into water. Two sets of samples were then isothermally aged at 300℃-500℃for different times up to 6000 h. The Vickers microhardness was measured by microhardness tester (HV-10) with a load for 5 kg for 15 s. The precipitation of Cu-rich clusters and crystal structural evolution of Cu-rich precipitates in RPV model steel were investigated by means of atom probe tomography (APT), extraction replica, EDS and HRTEM. The main conclusions are described as follows.(1) Effect of the different heat treatments before aging on the precipitation of Cu-rich clusters in RPV model steels.The analysis of APT revealed that smaller Cu-rich clusters were observed in RPV model steels aged for 100 h at 400℃after quenching. The number density of Cu-rich clusters was estimated to be 1.69×1023 /m~3. The number density of Cu-rich clusters increased to 6.23×1023 /m~3 after quenching and aging at 400℃for 300 h. After quenching and tempering, there were no Cu-rich clusters precipitated in the samples aged for 100 h at 400℃. Cu-rich clusters were observed in the samples aged for 1000 h at 400℃and the number density was estimated to be 6×1022 /m~3. The number density of clusters is approximately an order of magnitude lower in the samples after quenching and tempering compared with the samples after quenching. Higher dislocation density in the martensite after quenching could promote the precipitation of Cu-rich clusters.(2) Effect of Ni and Mn on the precipitation of Cu-rich clusters in RPV model steels.Besides at the interfaces and the dislocations, Ni-rich clusters could also act as the nucleation sites for the precipitation of Cu-rich clusters. The Cu content increases and the Ni content decreases at the central cores with increasing Cu atoms congregation, Ni and Mn atoms segregation on the exterior side of the cluster/matrix interface is also evident. Ni-rich clusters would act as the nucleation sites during the precipitation of Cu-rich clusters. Therefore the increase of Ni content in RPV steels could promote the precipitation of Cu-rich clusters. This is the essential reason that the presence of Ni in RPV steel could increase its sensitivity to neutron irradiation embrittlement.(3) Crystal structural evolution of Cu-rich nano phase in the samples with aging at 370℃for different aging time after quenching and tempering.There are 26, 31, 52 and 56 precipitates on the extraction replicas to be collected for samples aged for 1000, 3000, 4500 and 6000 h at 370℃after quenching and tempering, respectively. It can be seen that the average size of these nano phases was found to the range from 11 to 20 nm, and the number of precipitates is increased by longer aging. Most of these Cu-rich nano phases were found to be roughly spherical, but elongated ribbons have also been observed in the samples aged for 6000 h.The different Cu-rich nano phases have different Cu content and crystal structure, for example, some are 9R and some are fcc, but no distinct correlation with the size of Cu-rich nano phase. Twinned structure can often be observed in fcc Cu-rich nano phase.(4) Crystal structural evolution and composition of Cu-rich nano phase in the samples with aging at 400℃for different aging time after quenching and temperingA Cu-rich nano phase with 20 nm diameter was observed in the sample aged for 2000 h at 400℃by extraction replica analysis and the average composition of the precipitate was 65.8 Cu-34.2 Fe (in at.%). It had been found in the present work that, besides the 9R structure occurring, there exit also 2H variant and stacking faults within a copper precipitate in aged samples (2000 h at 400℃). The IFFT pattern shows that the (001) plane of the 9R structure copper phase is not perpendicular to the (100) plane but rather has a relative orientation of about 86.9°. And the (001) plane of the 2H structure copper phase is perpendicular to the (010) plane. The lattice parameters of the 2H structure are estimated to be a = b = 0.254 nm and c = 0.417 nm. The 2H variant has a hexagonal unit cell with axial ratio c/a = 1.642.(5) Crystallographic study of fct Cu-rich nano phases transformationA Cu-rich nano phase with 27 nm diameter was observed in the sample aged for 2000 h at 400℃by extraction replica analysis and the average composition of the precipitate was 80.5 Cu-19.5 Fe (in at.%). The results by HRTEM, FFT and IFFT analyses revealed that a type of transition phase and a transition state that have not been reported so far occurred during the crystal structural evolution of Cu-rich nano phase. This transition state is a product duringε'-Cu transition toε''-Cu phase. Andε'-Cu andε''-Cu phases are two phases that possess the same crystal structure (fct) and the different lattice parameters.Through the theoretical calculation, it can be obtained that the lattice parameters of theε'-Cu nano phase are to be a = b = 0.410 nm and c = 0.415 nm. Theε'-Cu nano phase has an fct unit cell with axial ratio c/a = 1.01. Through the experimental studies, it can be obtained that the lattice parameters of theε''-Cu nano phase are to be a = b = 0.441 nm and c = 0.369 nm. Theε''-Cu nano phase has an fct unit cell with axial ratio c/a = 0.84. To obtainε''-Cu phase, theε'-Cu unit cell is expanded about 7.6% parallel to the a-axes and b-axes, and contracted about 11.1% along the c-axes.A crystallographic model of the phase transformation fromε'-Cu toε''-Cu phase based on the experimentally observed were established. Two steps of the phase transformation fromε'-Cu toε''-Cu phase was proposed as follows. The first step: the {110} planes is the invariant planes and simple shear movement of atoms on {110} plane along [11-2] direction occurs at the beginning of transformation, and the angle between [11-2] direction and [01-1] direction inε'-Cu is transformed into the angle ofε''-Cu from 29.9°to 32.9°. In the next step, an inhomogeneous lattice invariant deformation (lineal adjustment) produces a slipped that matches the observed shape. This model agrees well with the experimentally observed orientation relationship. |
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